1. Field of the Invention
The invention relates to a method of agglomerating fly ash into pellets in order to make it more compatible with the environment, particularly when in contact with surface water and ground water, so that the fly ash can be dumped, e.g. as landfill.
2. Description of the Prior Art
Fly ash is produced mainly by the burning of coal. If it is collected, fly ash can be manufactured into other products, but for the most part of it is simply dumped. This dumping takes place most often in old gravel pits or quarries. Generally, this form of dumping has been objected to due to the polluting of ground water and surface water. This pollution is caused by the lixiviation (leaching) of the fly ash, which is manifested in a rise in the pH of the surface water, the solution of heavy metals from the ash and the transport of sulphate ions into the ground water. The extent to which this takes place depends of course heavily on the composition of the fly ash, which can vary greatly from case to case. For the purpose of illustrating this phenomenon only, the following table presents an analysis of the occurrence of several elements in three different types of fly ash from the burning of different sorts of coal. It should be remembered that all these elements are present in the form of oxides or other compounds.
______________________________________ Al % 12.2 12.6 13.2 Ca % 3.2 3.2 4.1 Mg % 1.7 1.7 2.2 Si % 27.8 27.9 29.9 Ti % 0.48 0.54 0.60 Fe % 5.4 5.6 6.0 P.sub.2 O.sub.5 % 0.4 0.4 0.4 S % 0.28 0.30 0.31 C % 10.2 9.3 3.2 F % 0.024 0.024 0.025 Na % 0.64 0.62 0.49 K % 2.9 2.8 2.1 Sr % 0.08 0.08 0.08 Cr % 0.017 0.017 0.017 Cu % 0.016 0.016 0.018 Mn % 0.076 0.076 0.104 Pb % 0.019 0.019 0.018 Zn % 0.047 0.047 0.034 Ni % 0.015 0.016 0.018 Hg ppm 1.0 1.0 0.8 Cd ppm 4 1.0 3 ______________________________________
To illustrate the pollution which is caused by lixiviation of fly ash, 150 g of fly ash with the composition given in the right hand column of the table above was lixiviated at room temperature with 750 g of water of pH 8.5 while stirring continously. The pH rose to approximately 12 within about one or two minutes, rising after further stirring for ten to twenty minutes to a final value of approximately 12.5. One object of the present invention is to provide a limitation to this lixiviation in such a manner that in a similar test the pH will not rise to more than 8-1/2 to 9 after a much longer time.
It has been previously suggested to sinter fly ash into agglomerates on a travelling grate so that the total surface of the fly ash mass is drastically reduced and a final product is obtained which is less reactive chemically and hence may be termed immobilized. In this known method the firing temperature was in fact continuously held at approximately 1000.degree. C., or at most 1100.degree. C. In order to obtain sufficient cohesion the method also required substantial amounts of lime and bentonite to be added.
It has become apparent that the fly ash in the pellets which were obtained by the method is still not sufficiently immobilized to allow the pellets to be dumped freely. Particularly, an unacceptable amount of heavy metals goes into solution in the ground water in comparison with the surface water, with all the well known harmful consequences for the environment that this involves. It also has become apparent that products of this method show very large variations in homogeneity. Pellets in the upper layer of pellets on the travelling grate were not allowed to reach a temperature higher than for example 1070.degree. C., which meant they attained insufficient cohesion. During the subsequent transport of these pellets a high percentage of very fine and semi-fine material sieves off, and this material has to undergo another treatment.
Attempts to obtain harder pellets by this method, by raising the firing temperature, have so far failed because the pellets appear to sinter together in the bed on the travelling grate, and during the first phase of the hardening process the pellets already form an impermeable layer, which suffocates the entire firing process.
The prior art also contains a proposal (GS No. 1520335--FMC Corporation) for an alternative use of fly ash, also involving pelletization. The pellets are formed for a specific use in the generation of synthesis gas, in which the pellets are heated and rain down through the gasification zone to supply heat for the endothermic reaction. This specific use requires very particular properties of the pellets, especially high density and high resistance to spalling and attrition. The production of the pellets is therefore carefully controlled. In particular careful sizing of the fly ash (to finer than 200 mesh) is performed before the pellets are made and excess water is extracted from the pellets in a special process. A two stage heating process is described. In the first stage, which may last up to 16 hours (examples given range up from 0.5 hours where success is achieved if bentonite is included as a binder), carbon is removed from the pellets by conversion into gaseous compounds and iron oxides in the pellets are converted to --Fe.sub.2 O.sub.3. In a second heating stage, sintering is carried out.
This process with its careful preparation of the pellets and its long heating time is clearly wholly unsuitable for the large-scale treatment of fly ash to render it suitable for dumping, in which considerations of high uniform density and high strength are unimportant.